Cleanup fileuploader

using enums instead of static variables.
double check if instantUpload is authorized in FileUploader (actually this should be done by upload issuer).


Structural question:
Should not better FileUploader handle re-uploading in case of failed uploads (e.g., due to no connection and exeeded storage quota)? Right now only instantUploads are retried (this is done by InstantUploadBroadcastReceiver)